1. Field of the Invention
The present invention relates to a method for controlling a hydrophilic droplet using the principle of electrowetting, and more particularly, to a method for controlling the contact angle of the droplet in electrowetting and an apparatus for controlling the droplet formed by the method.
2. Description of the Related Art
Recently, there has been an increasing interest in a micro total analysis system (μTAS). A type of μTAS, lab-on-a-chip (LOC) has been studied for embodying on one chip all processes such as sampling, pre-treating and reacting the sample, and separating and analyzing the reacted sample.
One issue in the development of such an LOC is the control of a micro flow field for carrying out several processes within the LOC. Among the currently used methods to control the micro flow field in the LOC, are methods that employ a syringe pump, electroosmosis and HPLC (High Performance Liquid Chromatography), or the like, especially to control the transfer of the micro flow field. In this regard, the syringe pump, HPLC, and the like, have disadvantages in that the flow shows a pulsatory form. In addition, the costs are therefore increased. Electro-osmosis displays a slow ability to control the flow velocity and requires high voltage.
Meanwhile, surface tension is one of the forces that act on a fluid in the micro flow field. In one method for controlling the flow by using such surface tension, Lippmann proposes electrocapillarity that uses the capillary force change of the electrolyte solution while changing the metal surface by using an externally applied electrostatic charge. However, the flow field control method using such electrocapillarity has drawbacks in that the electricity may flow in the fluid and the surface conditions can be varied by electrochemical reaction brought on by the use of the electricity.
In order to overcome the drawback raised by the electrocapillarity, electrowetting, which controls the hydrophilic droplet by applying an external voltage is used. In electrowetting, the hydrophilic droplet is disposed upon a metal electrode coated with an insulating material, rather than contacting the hydrophilic droplet directly with the metal electrode.
The electrowetting using the surface tension of the hydrophilic droplet can effectively control the flowing of the droplet with a low voltage of several volts using a MEMS (micro electromechanical system). A micro electromechanical system is simple and economical to fabricate. Further, it is non-destructive and can save the sample for further use and analysis, when compared with other methods that employ flow control after filling a microchannel with a fluid.
U.S. Pat. No. 6,565,727 discloses a technique that continuously forms an electrode patch that changes the contact angle of the droplet using a simple electrowetting principle and applies a voltage to the electrode patch through an electric circuit so as to facilitate the distribution, mixing and transportation of the droplet. This patent allows each droplet that overcomes limitations of a consecutive fluid operation method used in the conventional microfluid system to be subjected to a dioxide process, thereby resulting in an operation method of low power and rapid speed.
Based on this patent, there have been developments for microfluid system operation methods using the more improved electrowetting principle applicable for LOC.
Furthermore, a liquid lens attracting public attention in CeBIT 2004 is characterized in that if a voltage is applied to a polar solvent and a nonpolar solvent using the electrowetting phenomenon, a certain curvature is formed on the contact surface between two fluids and thus the focus can be automatically adjusted depending on the distance from the object without the mechanical operation as in the conventional optical lens.
Such liquid lens camera module is similar to the eye lens of human being in that a thickness of the lens is changed automatically depending on the distance from the object to adjust the focus. Since the liquid lens allows for proximate photographing at a distance of 5 cm, it can recognize a fingerprint or character by a cellular phone. Moreover, because it only takes 2/100 second to adjust the focus and the cost of production is about 40% lower than that of the conventional optical lens, the liquid lens is expected as a promising alternative to the optical lens.
Meanwhile, the hydrophilic droplet used in the methods for controlling the droplet using the electrowetting principle hitherto known has a low electrowetting threshold voltage, thereby involving problems that the contact angle change of the droplet is not sufficiently wide and the velocity scope of the droplet is not so large.